Journal of Environment and Earth Science ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol. 3, No.7, 2013
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Table 1. Soil sample and organic wastes physicochemical and microbiological analysis Parameter Soil SD YP PD GD CD Organic carbon (%) 1.05±0.01 49.7±0.3 37.6 24.3 ± 0.02 22.2 ± 0.01 26.3 ± 0.01 Total nitrogen (%) 0.77±0.03 2.5 1.28 ± 0.02 2.70 ± 0.02 2.10 ± 0.03 2.40 ± 0.01 Carbon: Nitrogen 1.4:1 20:1 29:1 9:1 11:1 11:1 Phosphorus (%) 0.06 ± 0.02 0.035 0.044 ± 0.03 0.39 ± 0.01 0.23 ± 0.02 0.25 ± 0.03 pH 7 ± 0.1 5.2 ± 0.3 6.5 ± 0.2 7.2 ± 0.2 6.9 ± 0.1 8.4 ± 0.2 Moisture (%) 11.4 ± 0.02 9.4 ± 0.1 7.2 ± 0.2 5.0 ± 0.3 6.3 ± 0.2 8.4 ± 0.2 4 3 3 5 5 THDB (CFU/g) 0.1±0.8×10 0.1±0.1×10 0.2±0.2×10 0.3±2.3×10 0.1±0.7×10 0.3±1.7×105 Note. Each value is a mean of three replicates and ± indicates standard deviation among them. SD = Sawdust; YP =Yam peel; PD = Poultry dung; GD = Goat dung; CD = Cow dung 2.3. Solid-Phase Experimental Design and Soil Treatment Soil samples (1 kg) was put into 8 different plastic bins (microcosm) with a volume of about 3 L and labeled A to H, respectively. The soil in each plastic bins was spiked with 10% (w/w) diesel oil and thoroughly mixed together to achieve complete artificial contamination. 10% spiking was adopted in order to achieve severe contamination because above 3% concentration, oil has been reported to be increasingly deleterious to soil biota and crop growth (Osuji et al., 2005). Two weeks after contamination, the different remediation treatments were applied. The soil C:N ratio in each microcosm was adjusted by the addition of sawdust (SD), yam peel (YP), mixture or combination of cow dung (CD), goat dung (GD) and poultry dung) respectively, as carbon cosubstrate and nitrogen source and was thoroughly mixed (Table 2). It was assumed that the aforementioned quantities of the plant-residue organic wastes and animal-derived organic wastes applied to the relevant treatment microcosm were well worked to at least 15 cm depth in each plastic bin. Thus, the equivalents of 5000 kg per hectare of each amendment agents as single or in combinations were applied to each microcosm, respectively. These amounts of each organic waste supplied different amount of kg nitrogen per hectare (Table 2). The moisture content was adjusted to 50% water holding capacity by the addition of sterile distilled water and incubated at room temperature (28 ± 2 °C). The content of each bin was tilled twice a week for aeration, and the moisture content was maintained at 50% water holding capacity. The soil in Plastic bin A was autoclaved three times at 121oC for 30 min before contamination with diesel oil. The contaminated autoclaved soil in plastic bin A was without amendment agents and thus served as control. The experiment was set up in triplicate. In total, 24 microcosms were settled and incubated for six weeks (42 days). Periodic sampling from each plastic bin was carried out at 7-day intervals for 42 days to determine the residual total petroleum hydrocarbon (TPH) and hydrocarbon degrading bacteria, respectively. Table 2. Experimental design and types of organic wastes combination in different soil microcosm Microcosm Remediation Treatment and Description C:N Code A
1 kg Autoclaved Soil + 100 g Diesel oil
-
B
1 kg Soil + 100 g Diesel (Natural Attenuation)
-
C D
1 kg Soil + 100 g Diesel + 75 g SD 1 kg Soil + 100 g Diesel + 75 g YP
46:1 89:1
E
1 kg Soil + 100 g Diesel + 25 g CD + 25g GD + 25 g PD
16:1
F
1 kg Soil + 100 g Diesel + 37.5 g SD + 37.5g YP
10:1
G H
1 kg Soil + 100 g Diesel + 15 g SD + 15 g YP + 15 g CD + 15 g GD + 15 g PD 1 kg Soil + 100 g Diesel oil + 20 g of NPK fertilizer(20:10:10) 2.4 Total Petroleum Hydrocarbon Determination
30:1 20:1
The total petroleum hydrocarbon (TPH) content of the soil samples was determined gravimetrically by solvent extraction method of Adesodun and Mbagwu (2008). Soil samples (approximately 10 g) was taken from each microcosm and put into a 50-mL flask and 20 mL of n-hexane was added. The mixture was shaken vigorously on a magnetic stirrer for 30 minutes to allow the hexane extract the oil from the soil sample. The solution was then filtered using a Whatman filter paper and the liquid phase extract (filtrate) diluted by taking 1 mL of the extract into 50 mL of hexane. The absorbance of this solution was measured spectrophotometrically at a wavelength of 400 nm HACH DR/2010 Spectrophotometer using n-hexane as blank. The total petroleum 53